Earlier in the year I purchased a couple of these from Amazon. They retail at around £8
To be fair, I expected the quality to be shocking, but I was curious.
The footage below was taken a few months ago from the rear facing camera in my BMW. Although it's far from perfect, when the speed limit drops from a 40 to a 30 and the car behind gets a bit closer, you can read the number plate quite clearly.
When the car behind me, in the footage, gets frustrated with the cars in front travelling a shade under the speed limit, he overtakes a few of us at once in the 30, and you get some very clear footage of what's going on.
Whilst not the best quality, in the event of a collision I think it would be worth it's weight in gold at proving liability
And goes to show that a cheap one, is better than nothing.
https://www.youtube.com/watch?v=roJ-hteXQq0